This invention relates generally to magnetic field gradiometers and in particular to a fiber-optic multicomponent magnetic field gradiometer for measuring first, second, and higher order derivatives and to methods and apparatus for nulling and balancing the gradiometer.
In the detection, identification and location of tactical and strategic targets such as trucks, mines and submarines, one of the most difficult signatures to hide is the magnetic one. Even when visible methods are fogged over or radar methods are jammed, the magnetic signature indicating the existence and location of the targets still exists. Of particular interest is the location of targets whose exact position is unknown. Ferromagnetic hardware such as tanks and trucks on the ground, and submarines and mines under the water disturb the local geomagnetic field. These disturbances, or magnetic anomalies, are detectable as changes in the magnetic field measured as a sensor flies over them. For this reason, a sensor measuring the field changes or gradient provides more useful information than one merely measuring the field itself.
Present state-of-the-art for magnetic anomaly detection employs super-conducting quantum interference devices (SQUIDS) or proton precession devices. The latter devices do not have sufficient sensitivity to detect smaller targets such as trucks at sufficient range (1/2 kilometer) as a sensor flies over them, and require laborious calibration procedures. The former devices have a high resolution but, due to their superconducting nature, require large bulky devices for cooling.
K. P. Koo and G. H. Sigel, Jr. in "A Fiber-Optic Magnetic Gradiometer," (Journal of Light Wave Technology, Vol. Lt-1, No. 3, Sept., 1983) disclose a fiber-optic gradiometer capable of measuring both AC and DC magnetic field gradients and is incorporated herein by reference. The concepts disclosed are shown graphically in FIGS. 1-3. FIG. 1 shows the flight trajectory of a magnetic field gradiometer as it flies at a height Z of 100 meters over three 1-meter radius iron spheres 1, 2 and 3 located on the X-axis at -75, 0 and 75 meters, respectively. FIG. 2 shows a graph of the magnetic field disturbances along the X-axis caused by spheres 1, 2 and 3 with the circles 1a 2a and 3a indicating the respective spheres. FIG. 3 shows a graph of the first derivative or the magnetic field gradient of the magnetic field with spheres 1, 2 and 3 again indicated by circles 1a 2a and 3a respectively. However, as can be seen in FIGS. 1-3, no information on the location of the individual spheres in the array can be derived. However, in accordance with the teachings of the above referenced application Ser. No. 07/169,802, an accurate determination of the location of the target may be obtained from using the second derivative of the magnetic field as illustrated in FIG. 4.
The above teachings do not provide a mechanism for the simultaneous measurement of field gradients in plural directions, e.g. for the determination of all three magnetic field derivatives. Further, the above techniques do not permit a simplified DC nulling of the transducers such that the nulling may be done simultaneously for all bias coils. Moreover, the above techniques do not provide a teaching of a method or apparatus for simultaneously determining the second derivative for multicomponents of the gradient while utilizing a nulling technique which permits simultaneous nulling of the DC bias coils.
Accordingly, it is an object of the present invention to provide a method and apparatus capable of simultaneously determining the magnetic field and field derivatives, both first and second order, so as to permit accurate location of a target.
Yet another object of the invention is to provide a simplified nulling or balancing technique which can be used to simultaneously and independently null each of the DC bias coils of the magnetic transducers.
A further object of the invention is to apply the simplified nulling/balancing technique to the embodiment of multicomponent field gradient measurements so as to permit a greatly simplified optical connection of the component parts thereby reducing cost, size and complexity of the resulting interferometer.
It is a further object of the invention to provide an apparatus that is small and lightweight.
Other objects and advantages of the invention will become more apparent hereinafter in reference to the detailed description of the preferred embodiments and drawings.